Nicola,

Some potting soils do have fertilizers in them, I have used them before
myself when I was first starting out. They are really too light, so I added
clay to them from my yard. I live in Oklahoma and that is what most of my
soil is, clay.

When I repot my lilies I save the potting mix and use it over and over. I
put fertilizer tabs into the potting mix below the root system and fertilize
the plants every month.

The dirt it self doesn't bother the fish it just becomes part of their
environment. The nitrate levels are not affected by the dirt and the dirt
does not affect the nitrate levels.

HTH

Tom L.L.

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"Nicola B" wrote in message
...

ok thanks Tom, that helps. - but what I'm not sure about is, does all that
dirt bother the fish? does it raise the level of nitrates or nitrites?